Structure and order in soft matter: symmetry transcending length scale

Abstract

Molecular crystals, surfactant assemblies, and block copolymers are properly classified as “soft matter,” but these classes of materials are usually regarded as distinct owing to their different properties and applications. Furthermore, the length scales of translation order in typical molecular crystals are typically at least one order of magnitude smaller than in surfactant assemblies and block copolymers, which often organize as high symmetry hexagonal and cubic microstructures. Whereas the structures of molecular crystals are generally viewed to be governed by intermolecular forces at short length scales that impose local order, surfactant assemblies and block copolymers are locally disordered even though they form ordered microstructures at longer length scales. Though historically the crystal structures of most molecular crystals have been characterized for the purpose of understanding molecular structure, the recent emergence of crystal engineering as a solid-state discipline has shifted the focus toward the elucidation of the factors responsible for crystal packing and strategies for crystal design. This coincides with a growing interest in surfactants and block copolymers having materials properties that can be tuned precisely at the molecular level. As such, it seems opportune to begin examining the structural relationships that connect these classes of materials. The intent of this highlight is to provoke interest in such comparisons through examples of molecular crystals that exhibit features which mimic microstructures observed in surfactant assemblies and block copolymers. These compounds appear to be characterized by molecular components with some degree of amphiphilic character and supramolecular structural elements that (i) enforce topologies that are predestined to form certain high symmetry structures, (ii) introduce conformational softness that permits curvature at small length scales, (iii) form aggregates (i.e., through hydrogen bonding) which effectively increase the length scale so that curved surfaces required for high symmetry lattices can be formed with minimal energetic penalties. These examples suggest that the relationship between length scale, energy, curvature, and crystal symmetry bear further examination.